Many different species of fungi live harmlessly on the skin and inside the body of all animals including humans. However under certain circumstances, often as a result of a weakened or suppressed immune system, individuals will succumb to opportunistic fungal infections. The infections may be superficial or systemic. Superficial infections such as athletes' foot, ringworm and thrush are generally caused by pathogenic fungi. They are slow to develop, often remaining asymptomatic for many years, and are generally well tolerated or treated. Systemic infections are caused by primary and opportunistic fungi. They develop rapidly in susceptible hosts and hence require rapid treatment, often before diagnosis and tend to be associated with a poor prognosis. These usually occur in hospitalized patients.
The incidence of life-threatening fungal infections has increased dramatically as the population of immunocompromised individuals (including cancer, organ transplant and AIDS patients) has increased. As a result it is the cause of an increasing financial and logistic burden on the medical care system and its providers.
To date a number of different targets and metabolic pathways have been exploited clinically to kill fungi or inhibit their growth. The most recent approach to emerge targets the organisms' need to synthesise glucan. Glucan, in particular β-1,3-D-glucan, is essential to the integrity of the cell wall of a wide range of fungi, in particular the Candida and Aspergillus species. The main enzyme activity responsible for glucan synthesis within fungi is 1,3-β-D-glucan synthase. Inhibitors of this activity prevent the synthesis of glucan and thereby compromise the integrity of the cell wall, leading to destruction of the cell.
A class of compounds called echinocandins are known to act as glucan synthase inhibitors and include caspofungin acetate, micafungin and anidulafungin. The chemical structure of caspofungin acetate is shown below. The other echinocandin compounds have very similar structures.

Although the echinocandins have met some success in their antifungal action, it would be desirable to provide compounds that provide comparable glucan synthase inhibition and which have improved pharmacokinetic properties and which are relatively small organic molecules which are easier to manufacture.